1. Field of the Invention
The present invention relates to an apparatus and method for fabricating a polysilicon film for a semiconductor device.
2. Description of the Related Art
To manufacture a reliable semiconductor device:, one must keep the temperature of the apparatus uniform and minimize any contaminating particles.
Specifically, consider the case of forming a lower electrode with a hemispherical silicon grain (referred to as HSG-Si), in order to enlarge the electrostatic capacitance of a capacitor by increasing the area of the lower electrode. In this situation, it is critical to keep the temperature of the reaction chamber uniform and to keep the inside of the reaction chamber clean and free of contaminants.
Typically, to form the lower electrode with HSG-Si, a crystal growing step for forming crystal grains by migrating the amorphous silicon to the nucleus of crystalline silicon needs to be stable. Also, the speed of silicon surface migration for the growth of the crystal grain needs to be faster than the speed of the amorphous silicon crystallization in the lower amorphous silicon.
For the amorphous silicon to move toward the nucleus of the crystalline silicon, the amorphous silicon should have a free surface where the silicon atoms of the surface are not combined to any other atoms. When the surface is contaminated with other materials, the surface movement of the amorphous silicon atoms is impeded since the amorphous silicon atoms combine to the atoms of the other materials, thus making any further generation and growth of the nucleus impossible. Therefore, removing the surface contaminants on the wafer that is transferred to the reaction chamber, and keeping the inside of the chamber clean are important factors in semiconductor processing.
A general apparatus for fabricating the semiconductor device includes a cassette chamber in which a carrier having a wafer is loaded. The apparatus also contains a reaction chamber for performing a process, and a wafer cooling chamber after completing the process. A polyhedral transfer chamber having a robot arm is connected to the reaction chamber and cooling chambers for transferring the wafer to the respective chambers.
The structure of the reaction chamber is described as follows with reference to FIG. 1. A gate valve 31 that separates a wafer transfer chamber 10 and a reaction chamber 20 is disposed between the first side wall 30 of the reaction chamber and the wafer transfer chamber 10. A gas vent opening 33 is formed on a second side wall 32 opposite to the first side wall 30. A gas injection opening 35 is formed to pass through an upper wall 34 of the reaction chamber. Cooling jackets 40 and 42 are installed on the upper 34 and bottom 36 walls of the reaction chamber. A heating block 24, having a heater 22 and a susceptor 26 for sustaining a wafer 28 on the heating block 24, are installed inside the reaction chamber 20. Also, a turbo pump 38 is connected to the second side wall 32.
The operation of the apparatus of FIG. 1 will now be described. First, the wafer is transferred to the reaction chamber 20 after being transferred from the cassette chamber (not shown) and the wafer transfer chamber 10 by the robot arm. The pressure of the cassette chamber at the beginning of the transfer is about 1 mtorr.
However, air at a pressure of about 1 mtoor, which contains polluting particles, is also transferred from the cassette chamber to the wafer transfer chamber when the wafer is transferred. Therefore, the wafer transfer chamber is contaminated with the polluting particles. As a result, the reaction chamber 20 connected to the wafer transfer chamber 10 is also contaminated with the polluting particles. The surface of the wafer is thus contaminated by these polluting particles, such as moisture and carbon compounds, during the process of raising the temperature of the wafer 28 in the reaction chamber 20, thus reducing the reliability of the processing. Especially, in the case of forming the lower electrode with the HSG-Si, it is impossible to increase the surface area since the speed of the surface migration of the amorphous silicon is reduced by adsorption of contaminants to the amorphous silicon.
In the next steps, the surface of the wafer 28 is cleaned to remove an organic material or a native oxide film existing on the surface of the wafer prior to the processing in the reaction chamber 20. Therefore, a certain amount of moisture exists on the surface of the wafer 28 which is loaded in the cassette chamber (not shown) and the moisture is not completely evaporated and removed in the cassette chamber under the pressure of lmtorr. Therefore, vapor is continuously generated when the wafer 28 is transferred from the wafer transfer chamber 10 to the reaction chamber 20. Especially in a process for forming the HSG-Si, the speed of the surface migration of the amorphous silicon is reduced by the vapor which is continuously generated.
Typically, a cooling gas, such as argon or helium is injected into a cooling chamber (not shown) at a pressure of 1 to 100 torr. The cooling gas flows into the wafer transfer chamber 10 connected to the cooling chamber, and then flows into the reaction chamber 20, thus acting as a contaminant. As before, the speed of the surface migration of the amorphous silicon is reduced since the surface of the wafer 28 is contaminated by the cooling gas.
As shown in FIG. 1, the reaction chamber includes the cooling jackets 40 and 42 for keeping the temperature uniform on the upper and bottom walls 34 and 36 thereof. However, the temperature of the gate valve 31 separating the transfer chamber 10 and the reaction chamber 20, the first side wall 30 adjacent to the gate valve 31 and the second side wall 32 opposite to the first side wall 30, are all approximately 50.degree. C. or higher than the upper 34 and bottom 36 walls, since the above three portions have no cooling jackets. Thus, the surface contaminants existing on the chamber walls and the wafer may exude in a gas form from the gate valve 31, the first side wall 30, and the second side wall 32. Especially in the case of the process for forming the HSG-Si, it is impossible to achieve the desired surface increase effect since the exuded gas is adsorbed to the surface of the silicon.